Domestic appliances have been long known and used, which are equipped with steam production devices emitting jets for cleaning and disinfecting wall or furniture surfaces.
Particularly, these domestic appliances comprise a boiler which is filled with water and in which an electric resistor is immersed, which resistor is supplied with power and heats up, thereby heating water by conduction to an evaporation or heating temperature with hot water production.
A volume of steam is generated in the boiler, at a pressure higher than atmospheric pressure, which is controlled by a pressure safety valve or a similar device, that switches off the electric resistor each time that pressure reaches a maximum preset limit value, thereby stopping water heating and steam production for as long as is required to restore normal pressure values in the boiler.
The boiler has a fluid-tight connection for a steam ejecting pipe leading to an outflow control valve or a similar device, which is in turn equipped with a connection for the end of a steam carrying hose, for carrying the steam to be ejected to the surfaces to be cleaned.
The cyclic steam jets from the hose are controlled by the control valve which is actuated to open or close by special manual controls which are generally located on a handle of the hose, to be easily actuated by the users as needed.
Typically, boilers adapted to be mounted to these domestic appliances are composed of a box-like body, or boiler body, which defines therein a heating chamber having a considerable volume, and able to contain a correspondingly considerable volume of water, whereas the electric resistor is supported in the heating chamber to be entirely or almost entirely immersed in this volume of water for heating it.
These resistors usually have a rectilinear and substantially elongate shape, to be almost entirely immersed in the volume of water to be heated while occupying as small a space as possible, such that the boilers also have small dimensions, and do not increase the overall size of the appliances in which they are placed.
This prior art suffers from certain drawbacks.
A first drawback is that, in prior art boilers, the ratio of the total length of the radiant surface of the resistors to the volume of water to be heated is disadvantageous and does not afford high efficiency.
A further drawback is that this kind of linear resistors have a limited length, whereby the power supply exceeds the limits of the resistor surface area, which involves a risk of melting or failure of the resistors.
Furthermore, no quick and substantially smooth steam production can be obtained, namely because the radiant surface of the heating resistors is very small as compared with the boiler size, whereby heating and steam production times are long and discontinuous.
Another drawback is that heat exchange between the radiant surfaces of the electric resistors and the water to be heated occurs by simple direct contact therebetween, and no particular arrangement is provided for enhancing the heating effect of resistors or, assuming a target heating temperature, for reducing the power supply required to attain a target temperature and cause boiler water to evaporate and produce steam.
A further drawback is that, when the pressure safety valve cuts off power to the electric resistor as a predetermined pressure limit is reached in the heating chamber of the boiler due to steam generation, a considerable amount of the steam so produced shall be emptied for the pressure safety valve to restore power to the resistor, and hence start a new water heating cycle.
This adversely affects the overall efficiency of the domestic appliances with steam production because, while the boiler is being emptied of the steam by ejection thereof, with the resistor being powered off, a volume of cold refilling water is automatically introduced into the boiler, such cold water mixing with the water therein that has been heated by a previous heating cycle and is still in the heating chamber.
Therefore, the overall water temperature is decreased and parts of the power supplied to the resistor for heating are cyclically lost, which will increase the temperature drop that will be covered by the resistor, by heating again the water in the heating chamber once the pressure safety valve restores power for a subsequent heating and steam production step.
Another drawback is that prior art boilers have a large size and require accordingly large housings in the domestic appliances, whose design is affected by this requirement, with designers being limited in their ability to provide domestic appliances with steam production having a more pleasant appearance, improved ergonomic features, a lighter weight and easier storage even in small spaces, when not in use.